1. Field of the Invention
This invention relates to the field of tuned power amplifiers, particularly to tunable implementations of high efficiency switching power amplifiers.
2. Description of the Related Art
The demand for low-cost, reliable wireless communications continues to increase at a rapid rate, as do the demands on the technologies enabling such communications. Research is being conducted on many fronts to find ways to make the circuitry found inside devices such as cellular phones smaller, cheaper, easier to fabricate, less power-hungry, and more reliable.
A schematic diagram of a class E power amplifier is shown in FIG. 1. This type of amplifier is not commonly found in battery-powered communications devices in spite of its high efficiency, primarily because of its inherently narrow band nature. It has a theoretical efficiency of 100%, and actual efficiencies of better than 90% are routinely achieved. An RF input signal drives an active device 10, typically a transistor, which is operated as a switch. The transistor's current circuit is connected to drive a reactive load network 12 made up of a bias inductor L.sub.bias and a power source 14 connected in series and a shunt capacitor C.sub.sh connected across the transistor, and a series inductor L.sub.s and a series capacitor C.sub.s connected in series with the current circuit. The amplifier's RF output appears at C.sub.s 's other terminal, shown driving a load R.sub.load.
The class E amplifier shown is efficient only over a narrow frequency range centered around a specific operating frequency; i.e., it is inherently narrowband. The load network 12 is "tuned" to cause a particular phase relationship to exist at the operating frequency which allows the switching to occur at points of either zero voltage or zero current, leading to essentially lossless switching. The design equations used to achieve this condition are as described in U.S. Pat. No. 3,919,656 to Sokal et al.
There is a trade-off between the quality factor (Q) of the load network and the amplifier's bandwidth, with bandwidth being approximately inversely proportional to the network's Q. A very low Q yields an amplifier capable of high efficiencies over a wide bandwidth. However, a low Q also allows significantly higher amounts of harmonic currents to flow to the output load, so that the output requires additional filtering. Such filtering introduces additional losses, and requires additional reactive components which may have to be tuned to achieve acceptable performance
Due to the need for additional output filtering with a low Q load network, a high Q network is preferred. The high Q load and the very low power at which switching occurs combine to keep harmonic content low, simplifying the output filtering. Unfortunately, a high Q load network makes the amplifier efficient over a narrow band of frequencies, and the requisite phase relationship between the load network and the input frequency deteriorates rapidly as the input moves away from the designed operating point.
Similar trade-offs must be made when designing amplifiers from other classes in which an active device drives a reactive load, such as classes C, C-E, and F, with the result being that these amplifiers typically end up being narrowband. Though acceptable for use with inputs near the operating frequency, they are of little use where broad bandwidths, multiple frequencies, or frequency agility is a requirement, including most modern communications systems, both commercial and military.
Tunable inductors and/or capacitors are often incorporated into the design of a power amplifier, which are tuned by a technician to achieve acceptable performance when an amplifier is being assembled. This approach is not useful in a frequency hopping scheme, however, which requires the tuning of the load network to change in the field, to accommodate a change in operating frequency. Reactive components capable of being automatically tuned in the field exist, but either cannot be integrated with the other amplifier components, necessitating the use of hybrid or similar packaging schemes that increase size and weight while lowering efficiency and reliability, or provide low Q and a low efficiency amplifier. Reactive tuning elements are also problematic when designing a system for multiple frequency bands of operation, such as at 900 MHz and 1.9 GHz, or where extremely broad bandwidths are required In the prior art, such systems require lower Q matches, and suffer from reduced efficiency as a result.